1. Field of the Invention
The present invention relates to an arrangement of a piezoelectric hydrophone having a protective outer housing in which a water-proof closed casing is provided comprising two opposed and parallel diaphragm wafers which are both on their inner surfaces provided with piezoelectric crystals having metal coated surfaces, and where connecting wires extend from said casing through its hollow central part.
2. Prior Art Statement
The structure of hydrophones based on the above principle is known per se, and structures of this kind are disclosed, inter alia, in U.S. Pat. No. 3,255,431 and U.S. Pat. No. 3,970,878. The first mentioned specification discloses a design comprising an annular support member with a planar diaphragm element and a piezoelectric element provided on each side. In principle, the piezoelectric element may be provided inside and outside said diaphragm member. From said elements electric wires are extended, and the entire structure is provided in a case.
Another hydrophone embodiment, in principle using the same structure, is disclosed in U.S. Pat. No. 3,970,878. It is a major object of this patent to make the hydrophone element per se tight to prevent liquid/water from entering into the element and, thus, from contact with the piezoelectric crystals. The main embodiment in this respect is a structure, wherein the casing comprises two metal halves that are soldered together about a flange and wherein the electrical terminals extend through insulating glass members. In a special embodiment the central portion of the case is entirely manufactured from glass that is glued or bonded in any desired manner to the diaphragm wafers forming the side walls of the case. In such an embodiment sealing problems may arise at the joint edges.
Hydrophones of this kind are frequently used as receiver means in seismic explorations. For marine explorations a socalled "streamer" is provided having hydrophones placed inside a cable that may be filled with a special liquid, e.g. kerosene, or the hydrophones may be immediately exposed to sea water. Obviously, it is essential to prevent the piezoelectric crystal element from being exposed to the influence of the surrounding liquid. The hydrophone according to U.S. Pat. No. 3,255,431 does not comply with these requirements since, among others, leakage problems would arise in connection with the terminals. Special consideration was devoted to this problem in U.S. Pat. No. 3,970,878. The possibility of safeguarding the hydrophone against overpressure was discussed and the possibility of providing a spacer or a stiffening element inside the hydrophone casing was disclosed.
Recent development, however, requires higher hydrophone quality due to, among others, the development of digital streamer systems for utilization in seismic marine explorations. Such hydrophones should have an accuracy of approximately 2% with respect to the voltage and charging sensitivity against 5% previously in order to achieve a maximum data quality before the data reduction occurring in connection with the summation of the single hydrophones of an array.
Also, large numbers of such hydrohones are used in seismic explorations, and thus, it is essential that the cost of these products is kept as low as possible. Additionally, it is important that they are dependable.